In the field of evaluation of hydrocarbon-bearing subterranean rock formations, a downhole nuclear magnetic resonance (NMR) tool can be used to measure the nuclear magnetic properties of formation hydrogen. Core and log measurements include T2 decay presented as a distribution of T2 amplitudes versus time at each sample depth, typically from 0.3 ms to 3 s. NMR tool measured T2 distributions can be used to describe the fluids contained in a porous rock. It is useful to distinguish between movable and non-movable fluids. In the T2 scale, this can be done by applying a T2cutoff. If T2<T2cutoff, then the fluid is non-movable, if T2>T2cutoff, then the fluid is movable.
For some time in the oil industry, standard T2cutoff values have been used, such as 33 ms for clastics and 90 ms for carbonates. However, these values are frequently found unrealistic for log interpretation as a result of mineralogy dependent surface relaxivity effects which shift the T2 spectra. A known solution is to calibrate T2cutoff values with using laboratory NMR measurements on core samples.
However, the laboratory determination of T2cutoff using core samples and NMR measurements is a relatively long and expensive process. First, core samples are used, which are obtained from the formation. Second, an experimental procedure may be used that includes: (1) cleaning the sample; (2) 100% saturating the sample with water; (3) making NMR T2 measurements; (4) desaturating the sample by spinning in a centrifuge; and (5) making additional NMR T2 measurements. These NMR measurements are then used to determine the value of the T2cutoff. Furthermore, while a laboratory study can provide the correct values of T2cutoff to be used in log interpretation, it provides only values of T2cutoff at depths at which rock core samples were obtained and tested. Moreover, laboratory studies are generally performed at ambient conditions of temperature and pressure, therefore ignoring any potential effect of these parameters on the NMR response.